This invention relates to a process for manufacturing an electronic circuits wherein an electronic device or component (part) such as semiconductor integrated circuit (LSI) is connected to a circuit substrate, especially by soldering without using flux.
Soldering between a circuit substrate and a semiconductor integrated circuit (LSI) or the like has required that the surfaces of the metals in question to be joined should be kept clean and free from any material hindering wettability.
Plating has also required that the surface of the metal in question to be plated should be kept clean so that oxide film or layer or the like may not exist thereon.
When an Au wire or Au ribbon or the like is to be bonded on the surface of the metal in question by ultrasonic heat pressure technique, oxide film on the surface of the metal in question is also troublesome and therefore the surface of the metal in question must be kept clean.
The material hindering wettability of solder includes oxides, chlorides, sulfides, carbonates, various types of organic compounds, etc. Particularly, the most serious obstacle in the processes such as soldering, plating or ultrasonic heat pressure bonding of an Au wire or Au ribbon is oxide film present on the surface of the metal in question such as solder, nickel (Ni), nickel alloys (alloys of nickel with other substances).
This oxyde film is usually chemically dissolved by flux and converted into a liquid compound. Thus, the surface of the metal in question and metal atoms of solder get an opportunity to directly collide with each other to form a metallic bonding state sharing an outer electron shell, so that the metal in question and solder can be alloyed. However, flux residues still remain on the surface and must be cleaned off.
The presence of oxide film hinders plating. For example, oxide film acts as an insulating (dielectric) film to prevent electric conduction therethrough necessary for electroplating which is a typical example of plating and thus hinders such electroplating.
Oxide film also hinders replacement plating by preventing the reaction between the surface of the metal in question and a plating liquid.
In these plating processes, oxide film should be also removed by liquid treatment with hydrochloric acid or the like. However, residues still remain and are largely responsible for lowering the bonding reliability. Residues have been conventionally cleaned off with chlorofluorocarbons.
Recently, a technology has been developed to omit the step of cleaning off flux residues by using abietic acid (rosin) which leaves less amount of residues as flux and a minor amount of adipic acid or the like. However, the bonding reliability is insufficient.
This technology is described in detail in xe2x80x9cAlumit Technical Journal 19xe2x80x9d (1992) and xe2x80x9cAction Mechanism and Problems of Cleaning-Free Fluxxe2x80x9d by Kubota of Japan Industrial Technology Development Research Institute, Co.
A glazing method has also been proposed, which provides highly corrosion- and abrasion-resistant materials with very fine homogeneous texture or amorphous structure by irradiating metallic materials, steel materials, carbides or the like with laser beam. This glazing method is used for processing metallic materials to be subjected to high temperatures and high pressures such as materials for automobile turbines, and discussed in, for example, xe2x80x9cLaser Processing, Second Seriesxe2x80x9d, by A. Kobayashi, pp. 164, published by Kaihatsusha.
A known method for removing oxide film on the surfaces of metals without using flux or hydrochloric acid employs argon sputtering.
Japanese Patent Application Laying Open (KOKAI or Unexamined Publication) No. 63-97382 discloses a method for providing a highly adhesive pinhole-free coating film by plating a surface of, a metallic workpiece, roughened by blasting with an alloy element and thereafter melt processing the plated layer by irradiation with laser beam.
Further, Japanese Patent Application Laying Open (KOKAI) 62-256961 discloses a method for preparing a surface-treated layer with good corrosion resistance and solderability by forming an anodized layer on the surface of a base material formed of aluminium or alloys thereof.
As a result of reviews and studies of the above prior arts, the inventors have found the following problems.
(1) The method for removing oxide film by means of flux before soldering a circuit substrate and an integrated circuit involves the problem of necessarily requiring the step of cleaning off flux residues. Acid or the like remaining as residues may also cause the corrosion of metal.
In addition, a drying step is indispensable after cleaning.
(2) The method for removing oxide film by argon sputtering involves the problem of requiring a treatment in vacuum, with the result that the equipments can hardly be controlled and electronic devices or active elements of electronic devices may be unfavorably affected by argon sputter.
(3) The laser beam-madiated glazing method and the laser processing method disclosed in Japanese Patent Application Laying Open (KOKAI) 63-97382 will disadvantageously cause oxide film to grow during solidification of the metal surface because the both methods force the metal texture on the surface to melt by high energy laser beam in order to have abrasion resistance or denseness on the metal surface.
The surface treatment method disclosed in Japanese Patent Application Laying Open (KOKAI) 62-256961 can not be applied because it does not belong to the technique for removing oxide film.
The object of this invention is to solve the above problems of the prior arts and provide a process for manufacturing electronic circuits, according to which soldering can be carried out without using flux by applying a metal surface treatment procedure which allows oxide film, organic matters, carbon or the like on the surfaces of metals to be easily removed without complex process nor unfavorably affecting electronic devices, components or circuit substrates.
According to this invention, the above object is attained by a procedure of connecting an electronic device or component and a circuit substrate by means of solder, comprising the steps of irradiating said solder with laser beam to clean said solder, aligning or registering, and mounting said electronic device on said circuit substrate, and hot-melting said solder in a low-oxygen content atmosphere to bond said electronic device and said circuit substrate with each other.
In this procedure, the surface of a metal such as solder is irradiated with laser beam having a lower energy than the energy changing the texture of the surface of the metal. Thus, only the bonds between metal atoms and oxygen atoms on the surface of the metal are broken or released by the energy of the laser beam to remove oxide film as well as organic matters, carbon, etc. on the surface while the metal texture on the surface remains unmolten.
Oxide film on the surface of the metal can successfully be removed whether said laser beam irradiation takes place in any of the atmosphere such as air or He gas, or in vacuum.
Then, the electronic device or component and circuit substrate to be connected by solder are aligned or registered by a pre-setting liquid, and thereafter they are soldered together by hot-melting the solder from which has been removed oxide film in a low-oxygen content atmosphere. Thus, good soldering can be achieved without oxidizing soldered surfaces.